1. Field of the Invention
This invention generally relates to a process for the manufacture of a refractory monolithic lining of the wall and bottom of a ladle for casting and treating steel or for similar metallurgical vessels, as well as a device for the performance of the process. The invention also generally relates to a process for the partial reconstruction of the refractory lining in the upper zone of the wall area of a metallurgical vessel.
2. Background Information
Similar refractory linings of the prior art for metallurgical vessels generally include a permanent lining and a working lining. Fireclays or insulating bricks are conventionally used to construct the permanent lining, and are applied on the inside of the steel shell of the vessel. The working lining is applied on top of the permanent lining, and as it subsequently comes into contact with the molten steel, it is worn away by erosion, in particular as a result of the movement of the metal bath, and by chemical attack or erosion which is caused in particular by the slags which float on top of the molten metal.
The working lining can be constructed using refractory bricks or it can be installed in the form of a castable refractory mix which solidifies to form a monolithic layer. Combinations of these two types of linings are also possible.
In terms of the return on investment, taking into consideration both the cost of materials and useful life of the lining, as well as the time and human resources required to install the lining, a monolithic lining frequently turns out to be more economical than a lining which consists of refractory bricks. Since, as a rule, the amount of wear differs in different areas of the wall and bottom of the ladle, the lining is frequently divided into zones which have different thicknesses of the working lining and/or various grades of refractory materials which have different strengths or characteristics such as wear resistances.
Changes in modern methods of manufacturing steel have placed particularly stringent requirements on the refractory lining of casting and treatment ladles. In particular, after the transition to continuous casting, the higher tapping temperatures and the longer hold times of the molten steel in the ladle result in increased wear to the refractory lining.
After the ladle has been filled with molten steel and emptied numerous times, the working lining is either completely removed and replaced when it has reached the end of its useful life or, in the event of premature wear, it is repaired only in the damaged zones. The bottom frequently exhibits signs of wear sooner than the wall, in particular in the impact area of the casting stream and in the vicinity of the gas purging sets or ladle gates. In that case, the bottom lining must typically be removed and replaced several times during the life of the ladle. If, in the upper portion of the ladle, approximately 1 meter from the upper edge, the working lining is exposed to chemically corrosive slags, the lining in this slag zone must typically bq removed and replaced, or at least repaired, several times during the life of the ladle.
In accordance with a known lining method, the working lining of a steel casting ladle is constructed using refractory castables. This method requires a template which matches the internal contour of the finished, cast working lining. The castable refractory mix is poured into the space between the permanent lining and the template. To save installation time and increase the availability of the ladles, the bottom and the wall lining can be poured in a single process. In this manner, the bottom and the wall form a single, monolithic lining.
If only the wall lining is to be repaired, this method requires a template which matches the internal contour of the finished, cast working lining. The castable refractory mix is poured into the space between the permanent lining and the template.
Mixers, pumps and internal vibrators are generally used to perform the casting process. The casting should be performed continuously, over a period of about three hours for example, to prevent any hardening in the surface area. The vibrators are used to prevent the formation of cavities and voids and to achieve good densification. When the mix has set, e.g. after approximately 24 hours, the template can be removed.
When the wall and bottom are lined with a monolithic lining, and if premature wear occurs on the bottom, it has been found to be disadvantageous that, when the damaged portion of the bottom lining is removed, damage generally occurs to the rest of the bottom lining. Generally, the lower portion of the otherwise intact wall lining is damaged during the removal of the bottom lining, because the lining of the wall and of the bottom is a monolith. Consequently, that can mean that it is necessary to remove a major part of the wall lining prematurely.
To avoid problems in removing the bottom, the lining installation process can be performed so that a template is used which extends to the permanent lining of the bottom, and as a first step, only the wall lining is poured. After the castable refractory mix has set and the template has been removed, the bottom is poured in a second step. The result, when the bottom is later removed, is a design breaking point which prevents damage to the wall area of the lining. One disadvantage, however, is that additional time is required to allow the lining on the bottom to set before the ladle can be returned to operation, which means that the time it takes to line the vessel is almost doubled.
When a monolithic lining of the entire vessel wall is installed, one disadvantage is that if premature wear occurs in the area exposed to the slag, interim repairs to this area are so difficult that this area must continue to be lined with refractory bricks, as described in Stahl u. Eisen Special, October 1992, pp. 117-120, which is incorporated by reference herein.